1. Field of the Invention
The present invention relates generally to input devices for electronics and, more particularly, to a suspension for a touch sensitive input panel's or display's touch surface especially suited for use in eBook readers, cellular phones and personal digital assistants (PDAs), PC Tablets, as well as laptops, PCs, office equipment, medical equipment, TVs Monitors, or any other device that uses touch sensitive displays or panels.
2. Description of the Background
Touch sensitive screens or touch surfaces can detect the application of fingers and other passive objects. Touch surfaces are gaining in popularity and have been deployed in many products in recent years. A number of different technologies have been used to create touch surfaces, such as resistive, capacitive, infrared, surface acoustic wave (SAW), and others. Resistive pads, for example, comprise two conductive plates pressed together. The disadvantage of a resistive pad is that the resistive membrane material will wear out, initially resulting in further reduced clarity followed by dead spots. In addition, the production yield is typically rather poor, and the technology has a few disadvantages such as a fixed (non-user adjustable) actuation force and the light throughput through the resistive membranes is typically only around 70% to 75%.
Capacitive touch screens/pads operate by measuring the capacitance of the passive object to ground, or by measuring the alteration of the transcapacitance between different sensors. An example of a capacitive touchpad is described in U.S. Pat. No. 5,495,077 to Miller. Capacitive pads are relatively expensive to manufacture compared to resistive pads, and can only detect objects with sufficient capacitance. Small objects, such as the end of a regular stylus or pen, do not have enough capacitance to ground or transcapacitance to be detected by a capacitive touchpad. The actuation force can also not be set, and the force may be as low as a 0 gram force, that is, the touch screen registers a touch even before the user's finger touches the screen. This often leads to difficulties in implementing certain end-user features, such as handwriting recognition.
SAW devices emit sound along the surface of the pad and measure the interaction of the passive object with the sound. These devices work well, but are generally much too expensive for general applications. Infra red light based displays work in a similar fashion, but this technology typically adds a large size and price.
Finally, there are devices that use force sensors to measure the location and magnitude of the force exerted by the passive object on the touchpad. Force sensing technology is very interesting from both feature and cost perspectives. A force sensitive touchpad will sense force applied by any sort of passive object, regardless of the electrical conductivity or composition of the object. Such devices were originally described in U.S. Pat. No. 3,657,475 to Peronneau et al. and U.S. Pat. No. 4,121,049 to Roeber. These devices measure the forces transmitted by the touchpad to a fixed frame at multiple points, for example, at the corners of the pad. Roeber discloses a mathematical formula for deriving the position and magnitude of the force applied by a passive object from the forces measured at the multiple points.
For example, U.S. Pat. No. 4,511,760 to Garwin et al., issued Apr. 16, 1985, shows a force sensing data input device responding to the release of pressure force. The input surface is provided with a transparent faceplate mounted on force-sensing piezoelectric transducers. Preferably, four piezoelectric transducers are provided, one at each corner of a rectangular opening formed in the frame. To determine the point of application of force on the input surface, the outputs of the four transducers are first summed. To constitute a valid data entry attempt, the sum must exceed a first threshold while the user is pushing on the input surface. When the user releases his finger, a peak of the sum is detected, which is of opposite polarity from the polarity of the sum in the pushing direction. The individual outputs of the four sensors at the time that the peak of the sum occurs are used to calculate the point of application of the force.
United States Patent Application 2003/0085882 by Lu published May 8, 2003, shows a touch pad device having a support layer with a plurality of strain gauges in a matrix configuration. A touch layer is disposed on top of the strain gauge matrix. The touch layer is joined to the top of the strain gauge matrix. Sensor wires connect the strain gauges to a processor that is programmed with an algorithm to measure the location and pressure of simultaneous, multiple touches.
United States Patent Applications 2004/0108995 and 2004/0021643 both by Hoshino et al. show a display unit with touch panel mounted above a display via four differentially-mounted sensors. The pressure sensors detect force with which a pointing device such as a finger pushes the panel surface, in real time. The force with which the pointing device such as a finger pushes the panel surface is found from the following equation irrespective of the pointing position: P=a+b+c+d−a0+b0+c0+d0, which equation detects dragging of a cursor.
United States Patent Application 2005/0156901 by Ma et al., issued Jul. 21, 2005, shows a touch screen display system with a display screen and overlying touch surface. An imaging system determines an angular position on the touch surface of the object coming in contact with the touch surface.
United States Patent Application 2006/0016272 by Chang, published Jan. 26, 2006, shows a thin film touch pad with opposed sensor elements that generate an electrical signal that is proportional to both the applied pressure and the surface area at the location of the applied pressure. As a result of the complementary and overlapping orientation of these sensor elements, the first and second sensor elements generate an asymmetric pair of signals that uniquely define the applied pressure by position and magnitude.
U.S. Pat. No. 6,879,318 by Chan et al., issued Apr. 12, 2005, shows a touch screen mounting assembly for a liquid crystal display (LCD) panel including a bottom frame having a seated backlight panel and a plurality of mounted pressure-sensitive transducers, an LCD panel, and a top frame for exerting pressure when mounted to the bottom frame such that a plurality of compressible springs biases the LCD panel towards the bottom frame when touched or contacted by a user. The claims require the bottom and top frame assembly with backlight panel mounted therein on springs, and an overlying LCD panel.
The market success for force based touch screens and pads has so far been very limited for various reasons. Current implementations employ complex mechanical structures and appropriate force-sensing sensors. A method to overcome the mechanical complexities (promising a low cost and small size penalty) is described in International Application No. PCT US2008/003374 filed Mar. 14, 2008, which employs a figure-8 suspension concept to ensure that the touch screen will not move in the x-y plane. This is therein illustrated in FIG. 1 which shows the profile of how the wire or line (15) is wrapped around the touch surface (10) and the back surface (14) in a figure-8 loop. The line (15) is wrapped around all 4 sides, creating 4 separate figure-8 loops or one combined (using 4 or 1 lines). The line suspension can be designed in a number of variants, where the line may be wrapped around the surfaces as in FIG. 1, or it may secure the surfaces through hole or channels into the corner or the surfaces, or connected to wire holders which are fastened to the surfaces. Either way, the principles remains the same, the built-in tension in the line pulls the touch surface (10) towards the base plate (14) applying a stable, pre-loaded force onto the 4 force sensors (41). The forces in the string is also centering the touch plane (10) in relation to the base plate (14) as the forces in the x-y plane applied to the 4 corners are directed to the center of the touch plane. This centering force will allow the touch plane to remain centered and not move in the x-y plane, but allow for the small friction free movement required in the z-direction, typically less then 0.1 mm distance when using a piezo resistive sensor (other force sensors may require a larger z-direction movement).
The foregoing configuration provides a good mechanical suspension with a close to zero loss in the touch force, yet the device is free to move frictionless in the z-direction thereby ensuring that all of the touch force will be distributed to four force sensors. Moreover, the suspension is very low cost in terms of components. However, the assembly process is not optimal. In addition, it is difficult to apply a dust or water seal which typically requires a rubber or silicon material to touch the touch lens, and the seal must typically also be tailor made as well as tested and verified for each new design.
It would be greatly advantageous to provide a suspension solution that preserves the mechanical aspects of the force based touch screen system in a way where the mechanical characteristics of the system described above are maintained, but with a design that is optimized for both high volume production as well as for a built in low cost and high performing water seal. The same platform components should also be capable of accommodating different sensor sizes/shapes, should allow for a more cost efficient suspension mechanism, and should be reusable from one product implementation to the next.